ABSTRACT: In order to harness the potential of the HIV-specific antibody response for prevention, it is important to define the types of antibodies that block HIV infection or slow disease. Animal models are often used as a first step in defining proof-of-principle concepts and many elegant studies in non-human primate models of HIV support the potential of antibodies to provide protection from infection and disease. However, moving forward from these studies, which are conducted under very controlled situations and with a single viral strain, to understanding immune mechanisms of protection in the much more complex setting of natural HIV infection is challenging. It is really only possible to study immune correlates in humans in situations where there are HIV-exposed individuals who carry HIV-specific Abs where a subset remains uninfected while others succumb. This occurs in the setting of mother-to-child transmission (MTCT) of HIV, particularly postpartum infection, where maternal antibodies are passed to the infant and achieve their highest levels at birth. Thus, MTCT represents one of the few cases where the characteristics of protective antibodies can be studied for HIV. Our team has several remarkable cohort studies that uniquely position us to define immune correlates in MTCT. Our studies in the previous grant cycle showed an association between non-neutralizing Abs capable of antibody-dependent cellular cytotoxicity (ADCC) and infant outcomes. These studies represent one of the first to suggest a role for ADCC antibodies in protection in exposed humans and support findings of the RV144 HIV vaccine trial. Here we propose to build on these findings to more precisely define the target and function of the ADCC antibodies associated with better infant outcomes using an array of innovative methods, including systems serology and a new phage display method. We will further explore the nature of the ADCC activity using assays that measure ADCC in different ways. We will isolate ADCC antibodies from highly infectious non-transmitting mothers to begin to define details of the functional properties of protective antibodies and their origin. Because human studies are much more complex and subject to confounding influences compared to experimental studies, determining if the same correlates are identified in different cohorts is critical. Thus, this grant is design to take a broad approach to defining correlates of protection in a first cohort study followed by testing of identified correlates in a second cohort study. Overall, the proposed studies will leverage unique cohorts and exciting findings from the prior grant cycle to provide critical insight into the characteristics of antibodies that contribute to protection from HIV-infection in humans.